Rubber composition for tires and pneumatic tire

ABSTRACT

The present invention provides a rubber composition for treads, containing: a rubber component including an isoprene-based rubber and polybutadiene rubber; silica; carbon black; a terpene-based resin; and a water-soluble fine particle and/or a foaming agent, the terpene-based resin having a softening point of 70-150° C., an α-pinene unit content of 65-100% by mass, and a β-pinene unit content of 0-35% by mass, the rubber component including, based on 100% by mass thereof, 20-80% by mass of the isoprene-based rubber and 20-80% by mass of the polybutadiene rubber, with a combined amount of the isoprene-based rubber and the polybutadiene rubber of 90% by mass or more, the rubber composition containing, per 100 parts by mass of the rubber component, 20-200 parts by mass of the silica, 0.1-50 parts by mass of the carbon black, and 0.1-50 parts by mass of the terpene-based resin.

TECHNICAL FIELD

The present invention relates to a rubber composition for tires, and apneumatic tire including the rubber composition.

BACKGROUND ART

Studded tires or tire chains have been used for driving on snowy and icyroads. However, since they cause environmental problems such as dustpollution, studless winter tires have been proposed to replace them. Thematerials and structure of the studless winter tires are designed toallow the tires to be used on snowy or icy roads with rougher surfacesthan normal roads. For example, there have been developed rubbercompositions which contain diene rubbers having excellentlow-temperature properties, or which contain a large amount of softenersto enhance the softening effect (see, for example, Patent Literature 1).

From an environmental standpoint, it has recently been desirable tofurther provide good abrasion resistance and other properties inaddition to snow and ice performance. Thus, there is a need for abalanced improvement of these properties.

CITATION LIST Patent Literature

Patent Literature 1: JP 2009-091482 A

SUMMARY OF INVENTION Technical Problem

In this context, the present invention aims to provide a rubbercomposition for tires that provides a balanced improvement of snow andice performance, wet grip performance, abrasion resistance, and handlingstability, and a tire including the rubber composition.

Solution to Problem

The present invention relates to a rubber composition for treads,containing: a rubber component including an isoprene-based rubber andpolybutadiene rubber; silica; carbon black; a terpene-based resin; andat least one of a water-soluble fine particle or a foaming agent, theterpene-based resin having a softening point of 70 to 150° C., anα-pinene unit content of 65 to 100% by mass, and a β-pinene unit contentof 0 to 35% by mass, the rubber component including, based on 100% bymass thereof, 20 to 80% by mass of the isoprene-based rubber and 20 to80% by mass of the polybutadiene rubber, with a combined amount of theisoprene-based rubber and the polybutadiene rubber of 90% by mass ormore, the rubber composition containing, per 100 parts by mass of therubber component, 20 to 200 parts by mass of the silica, 0.1 to 50 partsby mass of the carbon black, and 0.1 to 50 parts by mass of theterpene-based resin.

Preferably, the terpene-based resin is at least one selected from thegroup consisting of α-pinene homopolymers, terpene-based resins havingan α-pinene unit content of 99% by mass or higher, and copolymerscontaining an α-pinene unit and a β-pinene unit.

Preferably, the water-soluble fine particle has a median particle sizeof 0.1 to 500 μm.

Preferably, the water-soluble fine particle is at least one selectedfrom the group consisting of magnesium sulfate, potassium sulfate,sodium chloride, calcium chloride, magnesium chloride, potassiumcarbonate, sodium carbonate, lignin derivatives, and saccharides.

Preferably, the polybutadiene rubber has a cis content of 90% by mass orhigher.

Preferably, the polybutadiene rubber is a modified polybutadiene rubberthat has been modified with a polar group interactive with silica.

Preferably, the silica has a nitrogen adsorption specific surface areaof 160 to 300 m²/g.

Preferably, the rubber composition has an E* at 0° C. of 3.0 to 8.0 anda difference between E* at −10° C. and E* at 10° C. of 7.0 or less.

Another aspect of the present invention relates to a pneumatic tire,including a tread, the tread containing the rubber composition.

Preferably, the tread in the pneumatic tire has a surface with poreshaving a pore size of 300 μm or less.

Preferably, the pneumatic tire is a studless winter tire.

Advantageous Effects of Invention

The rubber composition for treads of the present invention containspredetermined amounts of a rubber component including an isoprene-basedrubber and polybutadiene rubber, silica, carbon black, a specificterpene-based resin, and a water-soluble fine particle and/or a foamingagent. Such a rubber composition provides a balanced improvement of snowand ice performance, wet grip performance, abrasion resistance, andhandling stability.

DESCRIPTION OF EMBODIMENTS

The rubber composition for treads contains predetermined amounts of arubber component including an isoprene-based rubber and polybutadienerubber, silica, carbon black, a terpene-based resin having a softeningpoint of 70 to 150° C., an α-pinene unit content of 65 to 100% by mass,and a β-pinene unit content of 0 to 35% by mass, and a water-solublefine particle and/or a foaming agent. The combined use of the specificrubber component including predetermined amounts of an isoprene-basedrubber and polybutadiene rubber with the specific terpene-based resinand the water-soluble fine particle and/or foaming agent (at least oneselected from the group consisting of the water-soluble fine particleand the foaming agent) provides a balanced improvement of snow and iceperformance, wet grip performance, abrasion resistance, and handlingstability.

The reason for this effect is not clear, but it is believed that it maybe due to the following mechanism.

The specific terpene-based resin has notably high compatibility withisoprene-based rubbers. Thus, it is believed that even when this resinis incorporated into a soft rubber composition, e.g. for use in wintertires, it will improve snow and ice performance without reducingabrasion resistance. It is also believed that when the terpene-basedresin has a molecular weight (e.g. Mn, Mz, Mw) controlled within apredetermined range, it has further improved compatibility withisoprene-based rubbers, which makes it possible to prevent a decrease inperformance due to blooming and other issues and thus to maintain snowand ice performance for a long period of time.

Moreover, it is believed that the use of a water-soluble fine particleand/or a foaming agent allows the resulting rubber composition to havepores (cells) which provide a water draining function and also lead toincreased roughness of the rubber surface, thereby improving abrasionresistance while improving wet grip performance and snow and iceperformance. Moreover, despite the fact that such cell- or pore-formingrubber compositions tend to show extremely reduced abrasion resistanceas compared to those which have no cell (pore)-forming ability, thepresent invention makes it possible to improve wet grip performance andsnow and ice performance while maintaining abrasion resistance by usingthe specific terpene-based resin. This is probably because theterpene-based resin that is based primarily on an α-pinene monomerenhances rubber flexibility, provides high affinity for rubber, and canalso co-cure with rubber, thereby improving abrasion resistance, andalso because such a α-pinene monomer-based resin has steric propertiesthat are suitable particularly for improving snow and ice performanceand abrasion resistance, even as compared to conventional limonene orρ-pinene resins.

Therefore, the rubber composition provides a significantly improvedbalance of snow and ice performance, wet grip performance, abrasionresistance, and handling stability and further can provide goodlong-term maintenance (durability) of these properties.

(Rubber Component)

The rubber composition contains a rubber component including anisoprene-based rubber and polybutadiene rubber.

Examples of the isoprene-based rubber include natural rubber (NR),polyisoprene rubber (IR), refined NR, modified NR, and modified IR. TheNR and IR may be those commonly used in the tire industry. Examples ofthe NR include SIR20, RSS#3, and TSR20, while examples of the IR includeIR2200. Examples of the refined NR include deproteinized natural rubber(DPNR) and highly purified natural rubber (UPNR). Examples of themodified NR include epoxidized natural rubber (ENR), hydrogenatednatural rubber (HNR), and grafted natural rubber. Examples of themodified IR include epoxidized polyisoprene rubber, hydrogenatedpolyisoprene rubber, and grafted polyisoprene rubber. Theseisoprene-based rubbers may be used alone or in combinations of two ormore.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the amount ofthe isoprene-based rubber based on 100% by mass of the rubber componentis 20% by mass or more, preferably 25% by mass or more, more preferably30% by mass or more. The amount is 80% by mass or less, preferably 70%by mass or less, more preferably 60% by mass or less.

Any polybutadiene rubber (BR) may be used, including, for example, thosecommonly used in the tire industry such as high-cis BR, BR containing1,2-syndiotactic polybutadiene crystals (SPB-containing BR),polybutadiene rubber synthesized using rare earth catalysts (rareearth-catalyzed BR), and polybutadiene rubber modified with tincompounds (tin-modified BR). For example, commercial products availablefrom Ube Industries, Ltd., JSR Corporation, Asahi Kasei Corporation, andZeon Corporation may be used as the BR. These may be used alone or incombinations of two or more.

For good snow and ice performance, abrasion resistance, and otherproperties, the BR preferably has a cis content of 90% by mass orhigher, more preferably 95% by mass or higher.

Herein, the cis content (cis-1,4 linkage content) is calculated fromsignal intensities measured by infrared absorption spectrometry or NMRanalysis.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the amount ofthe BR based on 100% by mass of the rubber component is 20% by mass ormore, preferably 30% by mass or more, more preferably 40% by mass ormore, still more preferably 50% by mass or more. The upper limit of theamount is 80% by mass or less, preferably 70% by mass or less, morepreferably 65% by mass or less.

The BR may be either unmodified BR or a modified BR.

The modified BR may be, for example, a BR having a polar groupinteractive with silica. For example, it may be a chain end-modified BRobtained by modifying at least one chain end of BR with a compound(modifier) having the polar group (i.e., a chain end-modified BRterminated with the polar group); a backbone-modified BR having thepolar group in the backbone; a backbone- and chain end-modified BRhaving the polar group in both the backbone and chain end (e.g., abackbone- and chain end-modified BR in which the backbone has the polargroup, and at least one chain end is modified with the modifier); or achain end-modified BR that has been modified (coupled) with apolyfunctional compound having two or more epoxy groups in the moleculeso that a hydroxyl or epoxy group is introduced.

Examples of the polar group include amino, amide, silyl, alkoxysilyl,isocyanate, imino, imidazole, urea, ether, carbonyl, oxycarbonyl,mercapto, sulfide, disulfide, sulfonyl, sulfinyl, thiocarbonyl,ammonium, imide, hydrazo, azo, diazo, carboxyl, nitrile, pyridyl,alkoxy, hydroxyl, oxy, and epoxy groups. These polar groups may besubstituted. Preferred among these are amino (preferably amino whosehydrogen atom is replaced with a C1-C6 alkyl group), alkoxy (preferablyC1-C6 alkoxy), and alkoxysilyl (preferably C1-C6 alkoxysilyl) groups.

The modified BR may suitably be, for example: (1) a BR modified with acompound (modifier) represented by the following formula:

wherein R¹, R², and R³ are the same or different and each represent analkyl, alkoxy, silyloxy, acetal, carboxyl (—COOH), or mercapto (—SH)group, or a derivative thereof; R⁴ and R⁵ are the same or different andeach represent a hydrogen atom or an alkyl group, and R⁴ and R⁵ may bejoined together to form a ring structure with the nitrogen atom; and nrepresents an integer.

The BR modified with the compound (modifier) of the above formula maysuitably be, for example, a BR obtained by modifying the polymerizingend (active terminal) of a solution-polymerized polybutadiene rubberwith the compound of the above formula, among others.

R¹, R², and R³ are each suitably an alkoxy group, preferably a C1-C8,more preferably C1-C4 alkoxy group; R⁴ and R⁵ are each suitably an alkylgroup, preferably a C1-C3 alkyl group; and n is preferably 1 to 5, morepreferably 2 to 4, still more preferably 3. When R⁴ and R⁵ are joinedtogether to form a ring structure with the nitrogen atom, the ringstructure is preferably a 4- to 8-membered ring. The term “alkoxy group”encompasses cycloalkoxy (e.g. cyclohexyloxy) and aryloxy (e.g. phenoxy,benzyloxy) groups.

Specific examples of the modifier include2-dimethylaminoethyltrimethoxysilane,3-dimethylaminopropyltrimethoxysilane,2-dimethylaminoethyltriethoxysilane,3-dimethylaminopropyltriethoxysilane,2-diethylaminoethyltrimethoxysilane,3-diethylaminopropyltrimethoxysilane,2-diethylaminoethyltriethoxysilane, and3-diethylaminopropyltriethoxysilane. Preferred among these are3-dimethylaminopropyltrimethoxysilane,3-dimethylaminopropyltriethoxysilane, and3-diethylaminopropyltrimethoxysilane. These may be used alone or incombinations of two or more.

The modified BR may also suitably be a BR modified with any of thefollowing compounds (modifiers), for example: polyglycidyl ethers ofpolyols such as ethylene glycol diglycidyl ether, glycerol triglycidylether, trimethylolethane triglycidyl ether, and trimethylolpropanetriglycidyl ether; polyglycidyl ethers of aromatic compounds having twoor more phenol groups such as diglycidylated bisphenol A; polyepoxycompounds such as 1,4-diglycidylbenzene, 1,3,5-triglycidylbenzene, andpolyepoxidized liquid polybutadiene; epoxy group-containing tertiaryamines such as 4,4′-diglycidyl-diphenylmethylamine and4,4′-diglycidyl-dibenzylmethylamine; diglycidylamino compounds such asdiglycidylaniline, N,N′-diglycidyl-4-glycidyloxyaniline,diglycidylorthotoluidine, tetraglycidyl meta-xylenediamine,tetraglycidylaminodiphenylmethane, tetraglycidyl-p-phenylenediamine,diglycidylaminomethylcyclohexane, andtetraglycidyl-1,3-bisaminomethylcyclohexane;

amino group-containing acid chlorides such asbis(l-methylpropyl)carbamyl chloride, 4-morpholinecarbonyl chloride,1-pyrrolidinecarbonyl chloride, N,N-dimethylcarbamic acid chloride, andN,N-diethylcarbamic acid chloride; epoxy group-containing silanecompounds such as 1,3-bis(glycidyloxypropyl)-tetramethyldisiloxane and(3-glycidyloxypropyl)-pentamethyldisiloxane;

sulfide group-containing silane compounds such as (trimethylsilyl)[3-(trimethoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(triethoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(tripropoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(tributoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(methyldimethoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(methyldiethoxysilyl)propyl]sulfide, (trimethylsilyl)[3-(methyldipropoxysilyl)propyl]sulfide, and (trimethylsilyl)[3-(methyldibutoxysilyl)propyl]sulfide;

N-substituted aziridine compounds such as ethylene imine and propyleneimine; alkoxysilanes such as methyltriethoxysilane; (thio)benzophenonecompounds containing an amino group and/or a substituted amino groupsuch as 4-N,N-dimethylaminobenzophenone,4-N,N-di-t-butylaminobenzophenone, 4-N,N-diphenylaminobenzophenone,4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone,4,4′-bis(diphenylamino)benzophenone, andN,N,N′,N′-bis(tetraethylamino)benzophenone; benzaldehyde compoundscontaining an amino group and/or a substituted amino group such as4-N,N-dimethylaminobenzaldehyde, 4-N,N-diphenylaminobenzaldehyde, and4-N,N-divinylaminobenzaldehyde; N-substituted pyrrolidones such asN-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, N-phenyl-2-pyrrolidone,N-t-butyl-2-pyrrolidone, and N-methyl-5-methyl-2-pyrrolidone;N-substituted piperidones such as N-methyl-2-piperidone,N-vinyl-2-piperidone, and N-phenyl-2-piperidone; N-substituted lactamssuch as N-methyl-ε-caprolactam, N-phenyl-ε-caprolactam,N-methyl-ω-laurilolactam, N-vinyl-ω-laurilolactam,N-methyl-β-propiolactam, and N-phenyl-β-propiolactam; and

N,N-bis(2,3-epoxypropoxy)aniline,4,4-methylene-bis(N,N-glycidylaniline),tris(2,3-epoxypropyl)-1,3,5-triazine-2,4,6-triones,N,N-diethylacetamide, N-methylmaleimide, N,N-diethyl urea,1,3-dimethylethylene urea, 1,3-divinylethylene urea,1,3-diethyl-2-imidazolidinone, 1-methyl-3-ethyl-2-imidazolidinone,4-N,N-dimethylaminoacetophenone, 4-N,N-diethylaminoacetophenone,1,3-bis(diphenylamino)-2-propanone, and1,7-bis(methylethylamino)-4-heptanone. The modified BR is preferably onemodified with an alkoxysilane, among others.

The modification with any of the compounds (modifiers) may be performedby known methods.

Another example of the modified BR may be: (2) one produced by a methodthat includes: modification step (A) of performing a modificationreaction to introduce an alkoxysilane compound having two or morereactive groups, including an alkoxysilyl group, into the activeterminal of a BR having an active terminal; and condensation step (B) ofperforming a condensation reaction of the residual group of thealkoxysilane compound introduced into the active terminal, in thepresence of a condensation catalyst containing at least one elementselected from the group consisting of the elements of Groups 4, 12, 13,14, and 15 of the periodic table, wherein the BR is produced bypolymerization in the presence of a catalyst composition mainlycontaining a mixture of the following components (a) to (c):

component (a): a lanthanoid-containing compound which contains at leastone element selected from the group consisting of lanthanoids, or areaction product obtained by reaction between the lanthanoid-containingcompound and a Lewis base;

component (b): at least one compound selected from the group consistingof aluminoxanes and organoaluminum compounds represented by the formula(1): AlR^(a)R^(b)R^(c) wherein R^(a) and R^(b) are the same or differentand each represent a C1-C10.hydrocarbon group or a hydrogen atom, andR^(c) is the same as or different from R^(a) or R^(b) and represents aC1-C10 hydrocarbon group; and

component (c): an iodine-containing compound which contains at least oneiodine atom in its molecular structure.

In other words, the modified BR (modified BR(I)) may be produced byperforming a modification reaction to introduce an alkoxysilane compoundinto the active terminal of a BR having an active terminal (BR(I)), andthen performing a condensation reaction of the residual group of thealkoxysilane compound introduced into the active terminal, in thepresence of a condensation catalyst containing at least one of theelements of Groups 4, 12, 13, 14, and 15 of the periodic table.

The modification step (A) includes performing a modification reaction tointroduce an alkoxysilane compound having two or more reactive groups,including an alkoxysilyl group, into the active terminal of a BR havingan active terminal (BR(I)).

The BR(I) may be, for example, a polymer having repeating units derivedfrom at least one monomer selected from the group consisting of1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene,1,3-hexadiene, and myrcene.

The BR(I) may be produced by polymerization in the presence or absenceof a solvent. The solvent for the polymerization (polymerizationsolvent) may be an inert organic solvent. Specific examples includeC4-C10 saturated aliphatic hydrocarbons such as butane, pentane, hexane,and heptane; C6-C20 saturated alicyclic hydrocarbons such ascyclopentane and cyclohexane; monoolefins such as 1-butene and 2-butene;aromatic hydrocarbons such as benzene, toluene, and xylene; andhalogenated hydrocarbons such as methylene chloride, chloroform, carbontetrachloride, trichloroethylene, perchloroethylene, 1,2-dichloroethane,chlorobenzene, bromobenzene, and chlorotoluene.

The polymerization temperature in the production of the BR(I) ispreferably −30 to 200° C., more preferably 0 to 150° C. Thepolymerization reaction may be carried out in any reaction mode, such asusing a batch reactor or continuously using a multistage continuousreactor or other devices. The polymerization solvent, if used,preferably has a monomer concentration of 5 to 50% by mass, morepreferably 7 to 35% by mass. Moreover, in view of efficiency in theproduction of the BR and in order to prevent deactivation of the BRhaving an active terminal, the polymerization system preferably containsas small an amount as possible of deactivating compounds such as oxygen,water, and carbon dioxide gas.

The BR(I) is produced by polymerization in the presence of a catalystcomposition (hereinafter, also referred to as “catalyst”) mainlycontaining a mixture of components (a) to (c).

The component (a) is a lanthanoid-containing compound which contains atleast one element selected from the group consisting of lanthanoids, ora reaction product obtained by reaction between thelanthanoid-containing compound and a Lewis base. Preferred among thelanthanoids are neodymium, praseodymium, cerium, lanthanum, gadolinium,and samarium, with neodymium being particularly preferred. Theselanthanoids may be used alone or in combinations of two or more.Specific examples of the lanthanoid-containing compound includelanthanoid carboxylates, alkoxides, f-diketone complexes, phosphates,and phosphites. Preferred among these are carboxylates or phosphates,with carboxylates being more preferred.

Specific examples of the lanthanoid carboxylates include carboxylatesrepresented by the formula (2): (R^(d)—COO)₃M wherein M represents alanthanoid and each R^(d) is the same or different and represents aC1-C20 hydrocarbon group. R^(d) in formula (2) is preferably a saturatedor unsaturated alkyl group and also preferably a linear, branched, orcyclic alkyl group. The carboxyl group is bound to a primary, secondary,or tertiary carbon atom. Specific examples include salts of octanoicacid, 2-ethylhexanoic acid, oleic acid, stearic acid, benzoic acid,naphthenic acid, and “versatic acid” (trade name, available from ShellChemicals, a carboxylic acid whose carboxyl group is bound to a tertiarycarbon atom). Preferred among these are salts of versatic acid,2-ethylhexanoic acid, and naphthenic acid.

Specific examples of the lanthanoid alkoxides include those representedby the formula (3): (R^(e)O)₃M wherein M represents a lanthanoid.Specific examples of the alkoxy group represented by “R^(e)O” in formula(3) include 2-ethyl-hexylalkoxy, oleylalkoxy, stearylalkoxy, phenoxy,and benzylalkoxy groups. Preferred among these are 2-ethyl-hexylalkoxyand benzylalkoxy groups.

Specific examples of the lanthanoid-β-diketone complexes includeacetylacetone complexes, benzoylacetone complexes, propionitrileacetonecomplexes, valerylacetone complexes, and ethylacetylacetone complexes.Preferred among these are acetylacetone complexes and ethylacetylacetonecomplexes.

Specific examples of the lanthanoid phosphates or phosphites includebis(2-ethylhexyl) phosphate, bis(1-methylheptyl) phosphate,bis(p-nonylphenyl) phosphate, bis(polyethyleneglycol-p-nonylphenyl)phosphate, (1-methylheptyl) (2-ethylhexyl) phosphate, (2-ethylhexyl)(p-nonylphenyl) phosphate, mono-2-ethylhexyl (2-ethylhexyl)phosphonate,mono-p-nonylphenyl (2-ethylhexyl)phosphonate, bis(2-ethylhexyl)phosphinic acid, bis(1-methylheptyl)phosphinic acid,bis(p-nonylphenyl)phosphinic acid, (1-methylheptyl)(2-ethylhexyl)phosphinic acid, and (2-ethylhexyl)(p-nonylphenyl)phosphinic acid salts. Preferred among these arebis(2-ethylhexyl) phosphate, bis(1-methylheptyl) phosphate,mono-2-ethylhexyl (2-ethylhexyl)phosphonate, andbis(2-ethylhexyl)phosphinic acid salts.

Among the above lanthanoid-containing compounds, neodymium phosphates orcarboxylates are particularly preferred, with neodymium versatate orneodymium 2-ethyl-hexanoate being most preferred.

In order to solubilize the lanthanoid-containing compound in a solventor stably store the compound for a long period of time, it is alsopreferred to mix the lanthanoid-containing compound with a Lewis base,or react the lanthanoid-containing compound with a Lewis base to give areaction product. The amount of the Lewis base per mol of the lanthanoidis preferably 0 to 30 mol, more preferably 1 to 10 mol. Specificexamples of the Lewis base include acetylacetone, tetrahydrofuran,pyridine, N,N-dimethylformamide, thiophene, diphenyl ether,triethylamine, organophosphorus compounds, and monohydric or dihydricalcohols. The above-mentioned components (a) may be used alone or incombinations of two or more.

The component (b) is at least one compound selected from the groupconsisting of aluminoxanes and organoaluminum compounds represented bythe formula (1): AlR^(a)R^(b)R^(c) wherein R^(a) and R^(b) are the sameor different and each represent a C1-C10 hydrocarbon group or a hydrogenatom, and R^(c) is the same as or different from R^(a) or R^(b) andrepresents a C1-C10 hydrocarbon group.

The term “aluminoxane” (hereinafter, also referred to as “alumoxane”)refers to a compound having a structure represented by the followingformula (4) or (5), and may include alumoxane association complexes asdisclosed in Fine Chemical, 23, (9), 5 (1994), J. Am. Chem. Soc., 115,4971 (1993), and J. Am. Chem. Soc., 117, 6465 (1995), all of which arehereby incorporated by reference in their entirety.

In formulas (4) and (5), each R⁶ is the same or different and representsa C1-C20 hydrocarbon group, and p represents an integer of 2 or larger.

Specific examples of R⁶ include methyl, ethyl, propyl, butyl, isobutyl,t-butyl, hexyl, isohexyl, octyl, and isooctyl groups. Preferred amongthese are methyl, ethyl, isobutyl, and t-butyl groups, with a methylgroup being particularly preferred.

The symbol p is preferably an integer of 4 to 100.

-   -   Specific examples of the alumoxanes include methylalumoxane        (hereinafter, also referred to as “MAO”), ethylalumoxane,        n-propylalumoxane, n-butylalumoxane, isobutylalumoxane,        t-butylalumoxane, hexylalumoxane, and isohexylalumoxane.        Preferred among these is MAO. The alumoxanes may be produced by        known methods, such as, for example, by adding a        trialkylaluminum or dialkylaluminum monochloride to an organic        solvent such as benzene, toluene, or xylene, and then adding        water, steam, steam-containing nitrogen gas, or a salt having        water of crystallization such as copper sulfate pentahydrate or        aluminum sulfate hexadecahydrate to react them. These alumoxanes        may be used alone or in combinations of two or more.    -   Specific examples of the organoaluminum compounds of formula (1)        include trimethylaluminum, triethylaluminum,        tri-n-propylaluminum, triisopropylaluminum, tri-n-butylaluminum,        triisobutylaluminum, tri-t-butylaluminum, tripentylaluminum,        trihexylaluminum, tricyclohexylaluminum, trioctylaluminum,        diethylaluminum hydride, di-n-propylaluminum hydride,        di-n-butylaluminum hydride, diisobutylaluminum hydride,        dihexylaluminum hydride, diisohexylaluminum hydride,        dioctylaluminum hydride, diisooctylaluminum hydride,        ethylaluminum dihydride, n-propylaluminum dihydride, and        isobutylaluminum dihydride. Preferred among these are        diisobutylaluminum hydride, triethylaluminum,        triisobutylaluminum, and diethylaluminum hydride, with        diisobutylaluminum hydride being particularly preferred. These        organoaluminum compounds may be used alone or in combinations of        two or more.

The component (c) is an iodine-containing compound which contains atleast one iodine atom in its molecular structure. The use of such aniodine-containing compound facilitates production of a BR having a ciscontent of 94% by mass or higher. The iodine-containing compound may beany compound that contains at least one iodine atom in its molecularstructure, and examples include iodine, trimethylsilyl iodide,diethylaluminum iodide, methyl iodide, butyl iodide, hexyl iodide, octyliodide, iodoform, diiodomethane, benzylidene iodide, beryllium iodide,magnesium iodide, calcium iodide, barium iodide, zinc iodide, cadmiumiodide, mercury iodide, manganese iodide, rhenium iodide, copper iodide,silver iodide, and gold iodide.

In particular, the iodine-containing compound is preferably a siliconiodide compound represented by the formula (6): R⁷ _(q)SiI_(4-q) whereineach R⁷ is the same or different and represents a C1-C20 hydrocarbongroup or a hydrogen atom, and q represents an integer of 0 to 3; ahydrocarbon iodide compound represented by the formula (7): R⁸_(r)I_(4-r) wherein each R⁸ is the same or different and represents aC1-C20 hydrocarbon group, and r represents an integer of 1 to 3; oriodine. Such silicon iodide compounds, hydrocarbon iodide compounds, andiodine are well soluble in organic solvents, and thus are easy to handleand useful for industrial production.

Specific examples of the silicon iodide compounds (compounds of formula(6)) include trimethylsilyl iodide, triethylsilyl iodide, anddimethylsilyl diiodo. Preferred among these is trimethylsilyl iodide.

Specific examples of the hydrocarbon iodide compounds (compounds offormula (7)) include methyl iodide, butyl iodide, hexyl iodide, octyliodide, iodoform, diiodomethane, and benzylidene iodide. Preferred amongthese are methyl iodide, iodoform, and diiodomethane.

Among these iodine-containing compounds, iodine, trimethylsilyl iodide,triethylsilyl iodide, dimethylsilyl diiodo, methyl iodide, iodoform, anddiiodomethane are particularly preferred, with trimethylsilyl iodidebeing most preferred. These iodine-containing compounds may be usedalone or in combinations of two or more.

The mixing ratio of the components (components (a) to (c)) may beappropriately selected as needed. For example, the amount of component(a) per 100 g of a conjugated diene compound is preferably 0.00001 to1.0 mmol, more preferably 0.0001 to 0.5 mmol.

The amount of the alumoxane used as component (b), may be defined as themolar ratio of component (a) to the aluminum (Al) contained in thealumoxane. The molar ratio of “component (a)” to “aluminum (Al)contained in alumoxane” is preferably 1:1 to 1:500, more preferably 1:3to 1:250, still more preferably 1:5 to 1:200.

The amount of the organoaluminum compound used as component (b), may bedefined as the molar ratio of component (a) to the organoaluminumcompound. The molar ratio of “component (a)” to “organoaluminumcompound” is preferably 1:1 to 1:700, more preferably 1:3 to 1:500.

The amount of component (c) may be defined as the molar ratio of theiodine atom contained in component (c) to component (a). The molar ratioof “iodine atom contained in component (c)” to “component (a)” ispreferably 0.5 to 3.0, more preferably 1.0 to 2.5, still more preferably1.2 to 2.0.

In addition to components (a) to (c), the catalyst preferably containsat least one compound selected from the group consisting of conjugateddiene compounds and non-conjugated diene compounds, as appropriate, inan amount of 1000 mol or less, more preferably of 3 to 1000 mol, stillmore preferably 5 to 300 mol per mol of component (a). The catalystcontaining at least one compound selected from the group consisting ofconjugated diene compounds and non-conjugated diene compounds has muchimproved catalytic activity and is thus preferred. Examples ofconjugated diene compounds that can be used include 1,3-butadiene andisoprene. Examples of the non-conjugated diene compounds includedivinylbenzene, diisopropenylbenzene, triisopropenylbenzene,1,4-vinylhexadiene, and ethylidene norbornene.

The catalyst composition mainly containing a mixture of components (a)to (c) may be prepared, for example, by reacting components (a) to (c)dissolved in a solvent, and optionally at least one compound selectedfrom the group consisting of conjugated diene compounds andnon-conjugated diene compounds. The components may be added in any orderin the preparation. However, in order to improve polymerization activityand reduce the induction period for initiation of polymerization, it ispreferred that the components be previously mixed, reacted, and aged.The aging temperature is preferably 0 to 100° C., more preferably 20 to80° C. The aging time is not particularly critical. Moreover, thecomponents may be brought into contact with each other in a productionline before being added to a polymerization reaction vessel. In thiscase, an aging time of at least 0.5 minutes is sufficient. The preparedcatalyst will be stable for several days.

The BR(I) to be used for preparing the modified BR(I) preferably has aratio of the weight average molecular weight (Mw) to the number averagemolecular weight (Mn) measured by gel permeation chromatography, i.e., amolecular weight distribution (Mw/Mn), of 3.5 or less, more preferably3.0 or less, still more preferably 2.5 or less. A molecular weightdistribution of more than 3.5 tends to lead to decreases in rubberproperties, including tensile properties and low heat build-upproperties. Moreover, the lower limit of the molecular weightdistribution is not particularly critical.

The term “molecular weight distribution (Mw/Mn)” refers to a valuecalculated as the ratio of weight average molecular weight to numberaverage molecular weight (weight average molecular weight/number averagemolecular weight). The weight average molecular weight of the BR ismeasured by gel permeation chromatography (GPC) calibrated withpolystyrene standards. The number average molecular weight of the BR ismeasured by GPC calibrated with polystyrene standards.

The vinyl content and cis content of the BR(I) may be easily controlledby adjusting the polymerization temperature. The Mw/Mn may also beeasily controlled by adjusting the molar ratio of components (a) to (c).

The BR(I) preferably has a Mooney viscosity at 100° C. (ML₁₊₄, 100° C.)within a range of 5 to 50, more preferably 10 to 40. The Mooneyviscosity may be easily controlled by adjusting the molar ratio ofcomponents (a) to (c).

The BR(I) preferably has a 1,2-vinyl bond content (1,2-vinyl content,vinyl content) of 0.5% by mass or lower, more preferably 0.4% by mass orlower, still more preferably 0.3% by mass or lower. The 1,2-vinyl bondcontent of the BR(I) is also preferably 0.001% by mass or higher, morepreferably 0.01% by mass or higher.

The 1,2-vinyl bond content of the BR is calculated from signalintensities measured by NMR analysis.

The alkoxysilane compound used in modification step (A) (hereinafter,also referred to as “modifier”) has two or more reactive groups,including an alkoxysilyl group. The type of reactive group other thanthe alkoxysilyl group is not particularly limited, but is preferably,for example, at least one functional group selected from the groupconsisting of (f) an epoxy group, (g) an isocyanate group, (h) acarbonyl group, and (i) a cyano group. The alkoxysilane compound may bein the form of a partial condensate or a mixture of the alkoxysilanecompound and the partial condensate.

The term “partial condensate” refers to an alkoxysilane compound inwhich some (i.e. not all) of SiOR (wherein OR represents an alkoxygroup) groups are joined by condensation to form SiOSi bonds. The BR tobe used in the modification reaction preferably has at least 10% livingpolymer chains.

Specific suitable examples of the alkoxysilane compound that contains(f) an epoxy group (hereinafter, also referred to as “epoxygroup-containing alkoxysilane compound”) include2-glycidoxyethyltrimethoxysilane, 2-glycidoxyethyltriethoxysilane,(2-glycidoxyethyl)methyldimethoxysilane,3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane,(3-glycidoxypropyl)methyldimethoxysilane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and2-(3,4-epoxycyclohexyl)ethyl(methyl)dimethoxysilane. More preferredamong these is 3-glycidoxypropyltrimethoxysilane or2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

Moreover, examples of the alkoxysilane compound that contains (g) anisocyanate group (hereinafter, also referred to as “isocyanategroup-containing alkoxysilane compound”) include3-isocyanatopropyltrimethoxysilane, 3-isocyanatopropyltriethoxysilane,3-isocyanatopropylmethyldiethoxysilane, and3-isocyanatopropyltriisopropoxysilane. Particularly preferred amongthese is 3-isocyanatopropyltrimethoxysilane.

Moreover, examples of the alkoxysilane compound that contains (h) acarbonyl group (hereinafter, also referred to as “carbonylgroup-containing alkoxysilane compound”) include3-methacryloyloxypropyltriethoxysilane,3-methacryloyloxypropyltrimethoxysilane,3-methacryloyloxypropylmethyldiethoxysilane, and3-methacryloyloxypropyltriisopropoxysilane. Among these,3-methacryloyloxypropyltrimethoxysilane is particularly preferred.

Furthermore, examples of the alkoxysilane compound that contains (i) acyano group (hereinafter, also referred to as “cyano group-containingalkoxysilane compound”) include 3-cyanopropyltriethoxysilane,3-cyanopropyltrimethoxysilane, 3-cyanopropylmethyldiethoxysilane, and3-cyanopropyltriisopropoxysilane. Particularly preferred among these is3-cyanopropyltrimethoxysilane.

Among these modifiers, 3-glycidoxypropyltrimethoxysilane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,3-isocyanatopropyltrimethoxysilane,3-methacryloyloxypropyltrimethoxysilane, and3-cyanopropyltrimethoxysilane are particularly preferred, with3-glycidoxypropyltrimethoxysilane being most preferred.

These modifiers may be used alone or in combinations of two or more.Partial condensates of the alkoxysilane compounds may also be used.

In modification step (A), the amount of the alkoxysilane compound permol of component (a) used in the modification reaction is preferably0.01 to 200 mol, more preferably 0.1 to 150 mol. The modifier may beadded in any manner, for example, all at once, in portions, orcontinuously. Preferably, it is added all at once.

The modification reaction is preferably performed in a solution. Thesolution used in the polymerization which contains unreacted monomersmay be directly used as this solution. The modification reaction may becarried out in any reaction mode, such as using a batch reactor orcontinuously using a multistage continuous reactor, inline mixer, orother devices. Moreover, the modification reaction is preferablyperformed after completion of the polymerization reaction and beforesolvent removal, water treatment, heat treatment, the proceduresnecessary for polymer isolation, and other operations.

The temperature of the modification reaction may be the same as thepolymerization temperature during the polymerization of the BR.Specifically, it is preferably 20 to 100° C., more preferably 30 to 90°C. The reaction time in the modification reaction is preferably fiveminutes to five hours, more preferably 15 minutes to one hour. Incondensation step (B), after the introduction of the alkoxysilanecompound residue into the active terminal of the polymer, knownantioxidants or reaction terminators may optionally be added.

In modification step (A), it is preferred to add, in addition to themodifier, an agent which can be consumed by a condensation reaction withthe alkoxysilane compound residue, i.e. the modifier introduced into theactive terminal, in condensation step (B). Specifically, it is preferredto add a functional group-introducing agent.

The functional group-introducing agent may be any compound thatsubstantially does not directly react with the active terminal butremains unreacted in the reaction system. For example, the functionalgroup-introducing agent is preferably an alkoxysilane compound that isdifferent from the alkoxysilane compound used as the modifier, i.e., analkoxysilane compound that contains at least one functional groupselected from the group consisting of (j) an amino group, (k) an iminogroup, and (1) a mercapto group. The alkoxysilane compound used as thefunctional group-introducing agent may be in the form of a partialcondensate or a mixture of the partial condensate and the alkoxysilanecompound used as the functional group-introducing agent which is not apartial condensate.

Specific examples of the functional group-introducing agent that is analkoxysilane compound containing (j) an amino group (hereinafter, alsoreferred to as “amino group-containing alkoxysilane compound”) include3-dimethylaminopropyl(triethoxy)silane,3-dimethylaminopropyl(trimethoxy)silane,3-diethylaminopropyl(triethoxy)silane,3-diethylaminopropyl(trimethoxy)silane,2-dimethylaminoethyl(triethoxy)silane,2-dimethylaminoethyl(trimethoxy)silane,3-dimethylaminopropyl(diethoxy)methylsilane,3-dibutylaminopropyl(triethoxy)silane, 3-aminopropyltrimethoxysilane,3-aminopropyltriethoxysilane, aminophenyltrimethoxysilane,aminophenyltriethoxysilane, 3-(N-methylamino)propyltrimethoxysilane,3-(N-methylamino)propyltriethoxysilane,3-(1-pyrrolidinyl)propyl(triethoxy)silane, and3-(l-pyrrolidinyl)propyl(trimethoxy)silane;N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propaneamine,N-(1-methylethylidene)-3-(triethoxysilyl)-1-propaneamine,N-ethylidene-3-(triethoxysilyl)-1-propaneamine,N-(l-methylpropylidene)-3-(triethoxysilyl)-1-propaneamine,N-(4-N,N-dimethylaminobenzylidene)-3-(triethoxysilyl)-1-propaneamine,and N-(cyclohexylidene)-3-(triethoxysilyl)-1-propaneamine, andtrimethoxysilyl, methyldiethoxysilyl, ethyldiethoxysilyl,methyldimethoxysilyl, or ethyldimethoxysilyl compounds corresponding tothe foregoing triethoxysilyl compounds. Particularly preferred amongthese are 3-diethylaminopropyl(triethoxy)silane,3-dimethylaminopropyl(triethoxy)silane, 3-aminopropyltriethoxysilane,N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propaneamine, andN-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propaneamine.

Moreover, suitable examples of the alkoxysilane compound containing (k)an imino group (hereinafter, also referred to as “imino group-containingalkoxysilane compound”) include3-(1-hexamethyleneimino)propyl(triethoxy)silane,3-(1-hexamethyleneimino)propyl(trimethoxy)silane,(1-hexamethyleneimino)methyl(trimethoxy)silane,(l-hexamethyleneimino)methyl(triethoxy)silane,2-(1-hexamethyleneimino)ethyl(triethoxy)silane,2-(1-hexamethyleneimino)ethyl(trimethoxy)silane,3-(1-heptamethyleneimino)propyl(triethoxy)silane,3-(1-dodecamethyleneimino)propyl(triethoxy)silane,3-(1-hexamethyleneimino)propyl(diethoxy)methylsilane, and3-(1-hexamethyleneimino)propyl(diethoxy)ethylsilane; and1-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole,1-[3-(trimethoxysilyl)propyl]-4,5-dihydroimidazole,3-[10-(triethoxysilyl)decyl]-4-oxazoline,N-(3-isopropoxysilylpropyl)-4,5-dihydroimidazole, andN-(3-methyldiethoxysilylpropyl)-4,5-dihydroimidazole. Among these,3-(1-hexamethyleneimino) propyl(triethoxy)silane,3-(1-hexamethyleneimino)propyl(trimethoxy)silane,(1-hexamethyleneimino)methyl(trimethoxy)silane,1-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole, and1-[3-(trimethoxysilyl)propyl]-4,5-dihydroimidazole are more preferred.

Moreover, examples of the alkoxysilane compound containing (1) amercapto group (hereinafter, also referred to as “mercaptogroup-containing alkoxysilane compound”) include3-mercaptopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane,2-mercaptoethyltriethoxysilane, 2-mercaptoethyltrimethoxysilane,3-mercaptopropyl(diethoxy)methylsilane,3-mercaptopropyl(monoethoxy)dimethylsilane,mercaptophenyltrimethoxysilane, and mercaptophenyltriethoxysilane.Particularly preferred among these is 3-mercaptopropyltriethoxysilane.

Among these functional group-introducing agents,3-diethylaminopropyl(triethoxy)silane,3-dimethylaminopropyl(triethoxy)silane, 3-aminopropyltriethoxysilane,3-(1-hexamethyleneimino)propyl(triethoxy)silane,N-(1-methylpropylidene)-3-(triethoxysilyl)-1-propaneamine,N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propaneamine,3-(1-hexamethyleneimino)propyl(trimethoxy)silane,(1-hexamethyleneimino)methyl(trimethoxy)silane,1-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole,1-[3-(trimethoxysilyl)propyl]-4,5-dihydroimidazole, and3-mercaptopropyltriethoxysilane are particularly preferred, with3-aminopropyltriethoxysilane being most preferred.

These functional group-introducing agents may be used alone or incombinations of two or more.

The amount of the alkoxysilane compound used as the functionalgroup-introducing agent, per mol of component (a) is preferably 0.01 to200 mol, more preferably 0.1 to 150 mol.

The functional group-introducing agent is preferably added after theintroduction of the alkoxysilane compound residue into the activeterminal of the BR(I) in modification step (A) and before the start ofthe condensation reaction in condensation step (B). If added after thestart of the condensation reaction, the functional group-introducingagent may not uniformly disperse, resulting in reduced catalyticperformance. Specifically, the functional group-introducing agent ispreferably added five minutes to five hours after the start of themodification reaction, more preferably 15 minutes to one hour after thestart of the modification reaction.

When the functional group-containing alkoxysilane compound is used asthe functional group-introducing agent, a modification reaction occursbetween the BR(I) having an active terminal and a substantiallystoichiometric amount of the modifier added to the reaction system,thereby introducing the alkoxysilyl group into substantially all activeterminals; further, the functional group-introducing agent is added,whereby the alkoxysilane compound residues are introduced in an amountmore than the equivalent amount of the active terminal of the BR.

In view of reaction efficiency, the condensation reaction betweenalkoxysilyl groups preferably occurs between a free alkoxysilanecompound and the alkoxysilyl group present at the end of the BR, oroptionally between the alkoxysilyl groups at the ends of the BRs. It isnot preferred to perform a reaction between free alkoxysilane compounds.Thus, when an alkoxysilane compound is further added as a functionalgroup-introducing agent, its alkoxysilyl group preferably has lowerhydrolyzability than the alkoxysilyl group introduced into the end ofthe BR.

For example, it is preferred to combine a compound containing atrimethoxysilyl group with high hydrolyzability as the alkoxysilanecompound to be reacted with the active terminal of the BR(I) with acompound containing an alkoxysilyl group (e.g. a triethoxysilyl group)with lower hydrolyzability than the trimethoxysilyl group-containingcompound as the alkoxysilane compound to be further added as afunctional group-introducing agent.

The condensation step (B) includes performing a condensation reaction ofthe residual group of the alkoxysilane compound introduced into theactive terminal, in the presence of a condensation catalyst containingat least one element selected from the group consisting of the elementsof Groups 4, 12, 13, 14, and 15 of the periodic table.

The condensation catalyst may be any condensation catalyst that containsat least one element selected from the group consisting of the elementsof Groups 4, 12, 13, 14, and 15 of the periodic table. Preferably, forexample, the condensation catalyst contains at least one elementselected from the group consisting of titanium (Ti) (Group 4), tin (Sn)(Group 14), zirconium (Zr) (Group 4), bismuth (Bi) (Group 15), andaluminum (Al) (Group 13).

Specific examples of the condensation catalyst that contains tin (Sn)include bis(n-octanoato)tin, bis(2-ethylhexanoato)tin, bis(laurato)tin,bis(naphthenato)tin, bis(stearato)tin, bis(oleato)tin, dibutyltindiacetate, dibutyltin di-n-octanoate, dibutyltin di-2-ethylhexanoate,dibutyltin dilaurate, dibutyltin maleate, dibutyltin bis(benzylmaleate),dibutyltin bis(2-ethylhexylmaleate), di-n-octyltin diacetate,di-n-octyltin di-n-octanoate, di-n-octyltin di-2-ethylhexanoate,di-n-octyltin dilaurate, di-n-octyltin maleate, di-n-octyltinbis(benzylmaleate), and di-n-octyltin bis(2-ethylhexylmaleate).

Examples of the condensation catalyst that contains zirconium (Zr)include tetraethoxyzirconium, tetra-n-propoxyzirconium,tetra-i-propoxyzirconium, tetra-n-butoxyzirconium,tetra-sec-butoxyzirconium, tetra-tert-butoxyzirconium,tetra(2-ethylhexyl oxide)zirconium, zirconium tributoxystearate,zirconium tributoxyacetylacetonate, zirconium dibutoxybis(acetylacetonate), zirconium tributoxyethylacetoacetate, zirconiumbutoxyacetylacetonate bis(ethylacetoacetate), zirconiumtetrakis(acetylacetonate), zirconium diacetylacetonatebis(ethylacetoacetate), bis(2-ethylhexanoato)zirconium oxide,bis(laurato)zirconium oxide, bis(naphthenato)zirconium oxide,bis(stearato)zirconium oxide, bis(oleato)zirconium oxide,bis(linoleato)zirconium oxide, tetrakis(2-ethylhexanoato)zirconium,tetrakis(laurato)zirconium, tetrakis(naphthenato)zirconium,tetrakis(stearato)zirconium, tetrakis(oleato)zirconium, andtetrakis(linoleato)zirconium.

Examples of the condensation catalyst that contains bismuth (Bi) includetris(2-ethylhexanoato)bismuth, tris(laurato)bismuth,tris(naphthenato)bismuth, tris(stearato)bismuth, tris(oleato)bismuth,and tris(linoleato)bismuth.

Examples of the condensation catalyst that contains aluminum (Al)include triethoxyaluminum, tri-n-propoxyaluminum, tri-i-propoxyaluminum,tri-n-butoxyaluminum, tri-sec-butoxyaluminum, tri-tert-butoxyaluminum,tri(2-ethylhexyl oxide)aluminum, aluminum dibutoxystearate, aluminumdibutoxyacetylacetonate, aluminum butoxy bis(acetylacetonate), aluminumdibutoxyethylacetoacetate, aluminum tris(acetylacetonate), aluminumtris(ethylacetoacetate), tris(2-ethylhexanoato)aluminum,tris(laurato)aluminum, tris(naphthenato)aluminum,tris(stearato)aluminum, tris(oleato)aluminum, andtris(linoleato)aluminum.

Examples of the condensation catalyst that contains titanium (Ti)include tetramethoxytitanium, tetraethoxytitanium,tetra-n-propoxytitanium, tetra-i-propoxytitanium,tetra-n-butoxytitanium, tetra-n-butoxytitanium oligomer,tetra-sec-butoxytitanium, tetra-tert-butoxytitanium, tetra(2-ethylhexyloxide) titanium, bis(octane dioleate)bis(2-ethylhexyl oxide)titanium,tetra(octane dioleate)titanium, titanium lactate, titanium dipropoxybis(triethanolaminate), titanium dibutoxy bis(triethanolaminate),titanium tributoxystearate, titanium tripropoxystearate, titaniumtripropoxyacetylacetonate, titanium dipropoxy bis(acetylacetonate),titanium tripropoxyethylacetoacetate, titanium propoxyacetylacetonatebis(ethylacetoacetate), titanium tributoxyacetylacetonate, titaniumdibutoxy bis(acetylacetonate), titanium tributoxyethylacetoacetate,titanium butoxyacetylacetonate bis(ethylacetoacetate), titaniumtetrakis(acetylacetonate), titanium diacetylacetonatebis(ethylacetoacetate), bis(2-ethylhexanoato)titanium oxide,bis(laurato)titanium oxide, bis(naphthenato)titanium oxide,bis(stearato)titanium oxide, bis(oleato)titanium oxide,bis(linoleato)titanium oxide, tetrakis(2-ethylhexanoato)titanium,tetrakis(laurato)titanium, tetrakis(naphthenato)titanium,tetrakis(stearato)titanium, tetrakis(oleato)titanium, andtetrakis(linoleato)titanium.

Among the above-mentioned condensation catalysts, titanium(Ti)-containing condensation catalysts are more preferred. Among suchtitanium (Ti)-containing condensation catalysts, alkoxides,carboxylates, and acetylacetonate complex salts of titanium (Ti) arestill more preferred, with tetra-i-propoxytitanium (tetraisopropyltitanate) being particularly preferred. The use of a titanium(Ti)-containing condensation catalyst can more effectively promote thecondensation reaction of the residue of the alkoxysilane compound usedas the modifier and the residue of the alkoxysilane compound used as thefunctional group-introducing agent.

As to the amount of the condensation catalyst, the number of moles ofthe above-mentioned compounds that may be used as the condensationcatalyst is preferably 0.1 to 10 mol, particularly preferably 0.3 to 5mol per mol of the total alkoxysilyl groups in the reaction system.

The condensation catalyst may be added before the modification reaction,but is preferably added after the modification reaction and before thestart of the condensation reaction. Specifically, the condensationcatalyst is preferably added five minutes to five hours after the startof the modification reaction, more preferably 15 minutes to one hourafter the start of the modification reaction.

The condensation reaction in condensation step (B) is preferablyperformed in an aqueous solution. The temperature during thecondensation reaction is preferably 85 to 180° C., more preferably 100to 170° C., particularly preferably 110 to 150° C.

The condensation reaction is preferably performed in an aqueous solutionwith a pH of 9 to 14, more preferably 10 to 12. When the aqueoussolution has a pH in the above-mentioned range, the condensationreaction can be promoted to improve the temporal stability of themodified BR(I).

The reaction time of the condensation reaction is preferably fiveminutes to 10 hours, more preferably about 15 minutes to five hours.Moreover, the pressure in the reaction system during the condensationreaction is preferably 0.01 to 20 MPa, more preferably 0.05 to 10 MPa.

The condensation reaction may be carried out in any reaction mode, suchas using a batch reactor or continuously using a multistage continuousreactor or other devices. Moreover, solvent removal may be performedsimultaneously with the condensation reaction.

After the condensation reaction is performed as described above, aconventional post treatment may be performed to obtain a target modifiedBR.

Another example of the modified BR may be: (3) a tin-modified BR.

The tin-modified BR is preferably, but not limited to, a tin-modifiedpolybutadiene rubber (BR) produced by polymerization using a lithiuminitiator and which has a tin atom content of 50 to 3000 ppm, a vinylcontent of 5 to 50% by mass, and a molecular weight distribution of 2 orless.

The tin-modified BR is preferably one prepared by polymerizing1,3-butadiene using a lithium initiator and then adding a tin compound,and which has a tin-carbon bond at the molecular end.

Examples of the lithium initiator include lithium compounds such asalkyllithiums and aryllithiums.

Examples of the tin compound include tin tetrachloride and butyltintrichloride.

The tin-modified BR preferably has a tin atom content of 50 ppm orhigher. The tin atom content is also preferably 3000 ppm or lower, morepreferably 300 ppm or lower.

The tin-modified BR preferably has a molecular weight distribution(Mw/Mn) of 2 or less. The lower limit of the molecular weightdistribution is not particularly critical, but is preferably 1 or more.The tin-modified BR preferably has a vinyl content of 5% by mass orhigher. The vinyl content is also preferably 50% by mass or lower, morepreferably 20% by mass or lower.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the combinedamount of the isoprene-based rubber and BR based on 100% by mass of therubber component in the rubber composition is 90% by mass or more,preferably 95% by mass or more, more preferably 100% by mass.

The rubber component of the rubber composition may include other rubbercomponent species as long as the effect is not impaired. Examples ofother rubber component species include diene rubbers such as styrenebutadiene rubber (SBR), acrylonitrile butadiene rubber (NBR),chloroprene rubber (CR), butyl rubber (IIR), andstyrene-isoprene-butadiene copolymer rubber (SIBR).

(Silica)

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the rubbercomposition contains silica as filler. Examples of the silica includedry silica (anhydrous silica) and wet silica (hydrous silica). Amongthese, wet silica is preferred because it contains a large number ofsilanol groups. For example, commercial products available from Degussa,Rhodia, Tosoh Silica Corporation, Solvay Japan, and Tokuyama Corporationmay be used. These may be used alone or in combinations of two or more.

The amount of the silica per 100 parts by mass of the rubber componentis 20 parts by mass or more, preferably 30 parts by mass or more, morepreferably 40 parts by mass or more. The upper limit of the amount is200 parts by mass or less, preferably 150 parts by mass or less, morepreferably 100 parts by mass or less, still more preferably 80 parts bymass or less. When the amount is within the range indicated above, thesilica tends to disperse well so that an excellent balance between snowand ice performance and abrasion resistance, and other properties can beachieved.

The silica preferably has a nitrogen adsorption specific surface area(N₂SA) of 160 m²/g or more, more preferably 165 m²/g or more. The upperlimit of the N₂SA of the silica is not particularly critical, but ispreferably 300 m²/g or less, more preferably 200 m²/g or less. When theN₂SA is within the range indicated above, the silica tends to dispersewell so that an excellent balance between snow and ice performance andabrasion resistance, and other properties can be achieved.

The N₂SA of the silica is measured by the BET method in accordance withASTM D3037-93.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the silicacontent based on 100% by mass in total of the silica and carbon black inthe rubber composition is preferably 50% by mass or more, morepreferably 80% by mass or more, still more preferably 90% by mass ormore.

(Silane Coupling Agent)

The rubber composition which contains silica preferably further containsa silane coupling agent.

Non-limiting examples of the silane coupling agent include sulfidesilane coupling agents such as bis(3-triethoxysilylpropyl)tetrasulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(4-triethoxysilylbutyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide,bis(2-triethoxysilylethyl)trisulfide,bis(4-trimethoxysilylbutyl)trisulfide,bis(3-triethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)disulfide,bis(4-triethoxysilylbutyl)disulfide,bis(3-trimethoxysilylpropyl)disulfide,bis(2-trimethoxysilylethyl)disulfide,bis(4-trimethoxysilylbutyl)disulfide,3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, and3-triethoxysilylpropyl methacrylate monosulfide; mercapto silanecoupling agents such as 3-mercaptopropyltrimethoxysilane,2-mercaptoethyltriethoxysilane, NXT and NXT-Z both available fromMomentive, and Si363 available from Evonik; vinyl silane coupling agentssuch as vinyltriethoxysilane and vinyltrimethoxysilane; amino silanecoupling agents such as 3-aminopropyltriethoxysilane and3-aminopropyltrimethoxysilane; glycidoxy silane coupling agents such asγ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane;nitro silane coupling agents such as 3-nitropropyltrimethoxysilane and3-nitropropyltriethoxysilane; and chloro silane coupling agents such as3-chloropropyltrimethoxysilane and 3-chloropropyltriethoxysilane. Forexample, commercial products available from Degussa, Momentive,Shin-Etsu Silicone, Tokyo Chemical Industry Co., Ltd., AZmax. Co., andDow Corning Toray Co., Ltd. may be used. These may be used alone or incombinations of two or more.

The amount of the silane coupling agent per 100 parts by mass of thesilica is preferably 3 parts by mass or more, more preferably 6 parts bymass or more. The amount is also preferably 20 parts by mass or less,more preferably 15 parts by mass or less, still more preferably 12 partsby mass or less, further preferably 10 parts by mass or less. When theamount is within the range indicated above, an effect commensurate withthe amount tends to be obtained, and a good balance of snow and iceperformance, wet grip performance, abrasion resistance, and handlingstability also tend to be achieved.

(Carbon Black)

In view of the balance of antistatic properties, wet grip performance,abrasion resistance, and handling stability, the rubber compositioncontains carbon black as filler. Non-limiting examples of the carbonblack include N134, N110, N220, N234, N219, N339, N330, N326, N351,N550, and N762. For example, commercial products available from AsahiCarbon Co., Ltd., Cabot Japan K.K., Tokai Carbon Co., Ltd., MitsubishiChemical Corporation, Lion Corporation, NSCC Carbon Co., Ltd., andColumbia Carbon may be used. These may be used alone or in combinationsof two or more.

The amount of the carbon black per 100 parts by mass of the rubbercomponent is 0.1 parts by mass or more, preferably 1 part by mass ormore, more preferably 3 parts by mass or more. Also, the amount is 50parts by mass or less, preferably 20 parts by mass or less, morepreferably 10 parts by mass or less, still more preferably 7 parts bymass or less. When the amount is within the range indicated above, thecarbon black tends to disperse well so that a good balance of snow andice performance, wet grip performance, abrasion resistance, fueleconomy, and handling stability can be achieved.

The carbon black preferably has a nitrogen adsorption specific surfacearea (N₂SA) of 50 m²/g or more, more preferably 80 m²/g or more, stillmore preferably 100 m²/g or more. The N₂SA is also preferably 200 m²/gor less, more preferably 150 m²/g or less, still more preferably 130m²/g or less. When the N₂SA is within the range indicated above, thecarbon black tends to disperse well so that a good balance of snow andice performance, wet grip performance, abrasion resistance, and handlingstability can be achieved.

The nitrogen adsorption specific surface area of the carbon black isdetermined in accordance with JIS K6217-2:2001.

(Terpene-Based Resin)

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, the rubbercomposition contains a terpene-based resin having predeterminedsoftening point and α- and β-pinene unit contents.

The amount of the terpene-based resin per 100 parts by mass of therubber component is 0.1 parts by mass or more, preferably 10 parts bymass or more, more preferably 15 parts by mass or more, still morepreferably 20 parts by mass or more. Also, the amount is 50 parts bymass or less, preferably 40 parts by mass or less. When the amount iswithin the range indicated above, a good balance of snow and iceperformance, wet grip performance, abrasion resistance, and handlingstability tends to be achieved.

The terpene-based resin has a softening point of 70 to 150° C. The lowerlimit of the softening point is preferably 100° C. or higher, morepreferably 110° C. or higher, while the upper limit is preferably 140°C. or lower, more preferably 135° C. or lower. The terpene-based resinhaving a softening point not lower than the lower limit tends to providegood abrasion resistance. The terpene-based resin having a softeningpoint not higher than the upper limit tends to provide goodprocessability.

The softening point is measured in accordance with ASTM D6090 (publishedin 1997).

The terpene-based resin preferably has a number average molecular weight(Mn) of 500 to 775. The lower limit of the Mn is more preferably 580 ormore, still more preferably 620 or more, while the upper limit is morepreferably 765 or less, still more preferably 755 or less. When the Mnis within the range indicated above, a good balance of snow and iceperformance, wet grip performance, abrasion resistance, and handlingstability tends to be achieved.

The terpene-based resin preferably has a z-average molecular weight (Mz)of 1300 to 1600. The lower limit of the Mz is more preferably 1310 ormore, still more preferably 1320 or more, while the upper limit is morepreferably 1570 or less, still more preferably 1550 or less. When the Mzis within the range indicated above, a good balance of snow and iceperformance, wet grip performance, abrasion resistance, and handlingstability tends to be achieved.

The terpene-based resin preferably has a weight average molecular weight(Mw) of 800 to 1100. When the Mw is within the range indicated above, agood balance of snow and ice performance, wet grip performance, abrasionresistance, and handling stability tends to be achieved.

The terpene-based resin preferably has a molecular weight distribution(Mw/Mn) of 1.30 to 1.70. When the Mw/Mn is within the range indicatedabove, a good balance of snow and ice performance, wet grip performance,abrasion resistance, and handling stability tends to be achieved.

The Mn, Mw, and Mz are measured by gel permeation/size exclusionchromatography (GPC-SEC) as set forth in ASTM D5296 (published in 2005).

The terpene-based resin preferably has a glass transition temperature(Tg) of 25 to 90° C. The lower limit of the Tg is more preferably 35° C.or higher, still more preferably 38° C. or higher, while the upper limitis more preferably 85° C. or lower, still more preferably 81° C. orlower. When the Tg is within the range indicated above, a good balanceof snow and ice performance, wet grip performance, abrasion resistance,and handling stability tends to be achieved.

The Tg is measured using a differential scanning calorimeter (SC Q2000available from TA Instruments) in accordance with ASTM D6604 (publishedin 2013).

The terpene-based resin preferably has a limonene unit content (amountof limonene units based on 100% by mass of the terpene-based resin) of10% by mass or lower, more preferably 5% by mass or lower, still morepreferably 1% by mass or lower, particularly preferably 0% by mass.

The terpene-based resin may be a homopolymer of a single terpene(terpene monomer), a copolymer of two or more terpenes, or a copolymerof at least one terpene and at least one additional monomer other thanterpenes.

The terpene used for forming the terpene-based resin has a basicmolecular formula represented by (C₅H₈)_(n) wherein n represents thenumber of linked isoprene units and is 2 or more. Suitable examples ofthe terpene include α-pinene, β-pinene, δ-3-carene, and β-phellandrene;pyrolysates of α-pinene, β-pinene, δ-3-carene, δ-2-carene, andterpinene; and combinations thereof. Among these, α-pinene or β-pineneis preferred, with α-pinene being more preferred.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, theterpene-based resin preferably has a terpene unit content (amount ofterpene units based on 100% by mass of the terpene-based resin) of 80%by mass or more, more preferably 90% by mass or more, still morepreferably 95% by mass or more, particularly preferably 99% by mass ormore. Such a terpene-based resin may be a homopolymer or copolymerhaving only terpene structural units.

The terpene-based resin may suitably be a polymer containing an α-pineneunit or a polymer containing an α-pinene unit and a β-pinene unit. Morespecifically, it may be, for example, an α-pinene homopolymer, aterpene-based resin having an α-pinene unit content of 99% by mass orhigher, or a copolymer containing an α-pinene unit and a β-pinene unit.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, theterpene-based resin has an α-pinene unit content (amount of α-pineneunits based on 100% by mass of the terpene-based resin) within a rangeof 65 to 100% by mass. The α-pinene unit content is preferably 70% bymass or higher, more preferably 80% by mass or higher, still morepreferably 90% by mass or higher, and may be 100% by mass.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, theterpene-based resin has a β-pinene unit content (amount of β-pineneunits based on 100% by mass of the terpene-based resin) within a rangeof 0 to 35% by mass. The β-pinene unit content is preferably 30% by massor lower, more preferably 20% by mass or lower, still more preferably15% by mass or lower, particularly preferably 10% by mass or lower, andmay be 0% by mass.

In view of the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability, theterpene-based resin preferably has a combined content of α- and β-pineneunits (combined amount of α- and β-pinene units based on 100% by mass ofthe terpene-based resin) of 80% by mass or higher, more preferably 90%by mass or higher, still more preferably 95% by mass or higher,particularly preferably 99% by mass or higher.

The terpene-based resin may be synthesized, for example, by cationicpolymerization of one or two or more terpene monomers using a Lewis acidcatalyst.

Non-limiting examples of the Lewis acid catalyst include metal halidessuch as BF₃, BBr₃, AlF₃, AlBr₃, TiCl₄, TiBr₄, TiL₄, FeCl₃, FeCl₂, SnCl₄,WCl₆, MoCl₅, ZrCl₄, SbCl₃, SbCl₅, TeCl₂, and ZnCl₂; metal alkylcompounds such as Et₃Al, Et₂AlCl₃, EtAlCl₂, Et₃Al₂Cl₃, (i-Bu)₃Al,(i-Bu)₂AlCl, (i-Bu)AlCl₂, Me₄Sn, Et₄Sn, Bu₄Sn, and Bu₃SnCl; and metalalkoxy compounds such as Al(OR)_(3-x)Cl_(x) and Ti(OR)_(4-y)Cl_(y)wherein R represents an alkyl group or an aryl group, x represents aninteger of 1 or 2, and y represents an integer of 1 to 3. The examplesalso include: (i) combinations of AlCl₃ and alkyl tertiary amines suchas trimethylamine, (ii) combinations of AlCl₃ and organosiliconcompounds such as trialkyl silicon halides, lower dialkyl phenyl siliconhalides, or hexaalkyldisiloxanes, (iii) combinations of AlCl₃ andorganogermanium halides such as trimethylgermanium chloride ortriethylgermanium ethoxide, and (iv) C1-C18 lower alkyl groups.

When the cationic polymerization is performed by solutionpolymerization, the solvent to be used may be any solvent that allowspolymerization of terpene monomers. Examples include halogenatedhydrocarbons, aromatic hydrocarbons, and aliphatic hydrocarbons.Specific examples include halogenated hydrocarbon solvents such asmethylene chloride, chloroform, 1,1-dichloromethane, 1,2-dichloroethane,n-propyl chloride, 1-chloro-n-butane, and 2-chloro-n-butane; aromatichydrocarbon solvents such as benzene, toluene, xylene, anisole, andnaphtha; and aliphatic hydrocarbon solvents such as pentane, hexane,heptane, octane, cyclopentane, cyclohexane, methylcyclohexane, andethylcyclohexane. The polymerization reaction may be carried out, forexample, at a temperature of −120 to 100° C., −80 to 80° C., or 5 to 50°C.

(Water-Soluble Fine Particle)

The water-soluble fine particle may be any fine particle soluble inwater. For example, materials having a water solubility of at least 1g/100 g of water at 20° C. may be used.

In view of ice performance and other properties, the water-soluble fineparticle preferably has a median particle size (median size, D50) of 0.1to 1 mm. The lower limit of the D50 is more preferably 1 μm or more,still more preferably 2 μm or more, while the upper limit is morepreferably 800 μm or less, still more preferably 500 μm or less.

Herein, the median particle size can be measured by laser diffraction.

The amount of the water-soluble fine particle per 100 parts by mass ofthe rubber component is preferably 1 part by mass or more, morepreferably 5 parts by mass or more, still more preferably 15 parts bymass or more, further preferably 20 parts by mass or more, particularlypreferably 25 parts by mass or more. When the amount is not less thanthe lower limit, good snow and ice performance tends to be obtained. Theamount is also preferably 100 parts by mass or less, more preferably 70parts by mass or less, still more preferably 60 parts by mass or less,particularly preferably 50 parts by mass or less. When the amount is notmore than the upper limit, good rubber properties such as abrasionresistance tend to be obtained.

Examples of the water-soluble fine particle include water-solubleinorganic salts and water-soluble organic substances. These may be usedalone or in combinations of two or more.

Examples of the water-soluble inorganic salts include metal sulfatessuch as magnesium sulfate and potassium sulfate; metal chlorides such aspotassium chloride, sodium chloride, calcium chloride, and magnesiumchloride; metal hydroxides such as potassium hydroxide and sodiumhydroxide; carbonates such as potassium carbonate and sodium carbonate;and phosphates such as sodium hydrogen phosphate and sodium dihydrogenphosphate.

Examples of the water-soluble organic substances include ligninderivatives and saccharides.

Suitable examples of the lignin derivatives include lignin sulfonic acidand lignosulfonates. The lignin derivatives may be prepared by a sulfitepulping method or a kraft pulping method.

Examples of the lignosulfonates include alkali metal salts, alkalineearth metal salts, ammonium salts, and alcohol amine salts of ligninsulfonic acid. Preferred among these are alkali metal salts (e.g.potassium or sodium salts) and alkaline earth metal salts (e.g. calcium,magnesium, lithium, or barium salts) of lignin sulfonic acid.

The lignin derivative preferably has a degree of sulfonation of 1.5 to8.0/OCH₃. Such a lignin derivative includes a lignin sulfonic acidand/or lignosulfonate in which lignin and/or a degradation productthereof is at least partially substituted with a sulfo group (sulfonegroup). The sulfo group of the lignin sulfonic acid may be unionized, orthe hydrogen atom of the sulfo group may be replaced by an ion such as ametal ion. The degree of sulfonation is more preferably 3.0 to 6.0/OCH₃.When the degree of sulfonation is within the range indicated above, goodice performance tends to be obtained.

The degree of sulfonation of the lignin derivative particle (ligninderivative that forms the particle) refers to the ratio of introducedsulfo groups calculated by the following equation:Degree of sulfonation (/OCH₃)=(S atoms (mol) of the sulfone groups inthe lignin derivative)/(the methoxyl groups (mol) in the ligninderivative).

The saccharide may be any monosaccharide, oligosaccharide, orpolysaccharide having any number of carbon atoms. Examples of suchmonosaccharides include trioses such as aldotriose and ketotriose;tetroses such as erythrose and threose; pentoses such as xylose andribose; hexoses such as mannose, allose, altrose, and glucose; andheptoses such as sedoheptulose. Examples of such oligosaccharidesinclude disaccharides such as sucrose and lactose; trisaccharides suchas raffinose and melezitose; tetrasaccharides such as acarbose andstachyose; and higher oligosaccharides such as xylooligosaccharide andcellooligosaccharide. Examples of such polysaccharides include glycogen,starch (amylose, amylopectin), cellulose, hemicellulose, dextrin, andglucan.

The water-soluble fine particle may suitably be at least one selectedfrom the group consisting of magnesium sulfate, potassium sulfate,sodium chloride, calcium chloride, magnesium chloride, potassiumcarbonate, sodium carbonate, lignin derivatives, and saccharides.

(Foaming Agent)

Any type of foaming agent may be used including those known in the tireindustry. Examples include azodicarbonamide (ADCA),dinitrosopentamethylenetetramine (DPT), dinitrosopentastyrenetetramine,benzenesulfonyl hydrazide derivatives, p,p′-oxybis(benzenesulfonylhydrazide) (OBSH), carbon dioxide-generating ammonium bicarbonate,sodium bicarbonate, ammonium carbonate, nitrogen-generatingnitrososulfonylazo compounds, N,N′-dimethyl-N,N′-dinitrosophthalamide,toluenesulfonyl hydrazide, p-toluenesulfonyl semicarbazide, andp,p′-oxybis(benzenesulfonyl semicarbazide). Among these,azodicarbonamide (ADCA) or dinitrosopentamethylenetetramine (DPT) ispreferred, with azodicarbonamide (ADCA) being more preferred.

The amount of the foaming agent per 100 parts by mass of the rubbercomponent is preferably 1 part by mass or more, more preferably 5 partsby mass or more. When the amount is not less than the lower limit, goodsnow and ice performance tends to be obtained. The amount is alsopreferably 20 parts by mass or less, more preferably 15 parts by mass orless. When the amount is not more than the upper limit, good rubberproperties such as abrasion resistance tend to be obtained.

A rubber vulcanizate obtained by vulcanizing the rubber compositioncontaining the foaming agent preferably has an expansion ratio of 0.1 to50%, more preferably 3 to 40%. When the expansion ratio is within therange indicated above, it is possible to ensure the formation of cellswhich can effectively function as drainage channels, while maintaining amoderate amount of cells, thereby avoiding risk of impairing durability.The expansion ratio of the rubber vulcanizate refers to an averageexpansion ratio (Vs), specifically calculated by the following equation(I):Vs=(ρ₀/ρ₁−1)×100(%)  (I)wherein pi represents the density (g/cm³) of the rubber vulcanizate(foamed rubber), and ρ₀ represents the density (g/cm³) of the solidphase of the rubber vulcanizate (foamed rubber).(Liquid Plasticizer)

In view of processability and the balance of snow and ice performance,wet grip performance, abrasion resistance, and handling stability, therubber composition preferably contains a liquid plasticizer. The term“liquid plasticizer” refers to a plasticizer that is liquid at roomtemperature (25° C.).

The amount of the liquid plasticizer per 100 parts by mass of the rubbercomponent is preferably 0.1 parts by mass or more, more preferably 10parts by mass or more, still more preferably 15 parts by mass or more.The amount is also preferably 100 parts by mass or less, more preferably60 parts by mass or less, still more preferably 40 parts by mass orless. When the amount is within the range indicated above, a goodbalance of snow and ice performance, wet grip performance, abrasionresistance, and handling stability tends to be achieved.

Examples of the liquid plasticizer include oils, liquid resins, andliquid diene polymers. These may be used alone or in combinations of twoor more.

Examples of the oils include process oils and plant oils, and mixturesthereof. Examples of the process oils include paraffinic process oils,aromatic process oils, and naphthenic process oils. Examples of theplant oils include castor oil, cotton seed oil, linseed oil, rapeseedoil, soybean oil, palm oil, coconut oil, peanut oil, rosin, pine oil,pine tar, tall oil, corn oil, rice oil, safflower oil, sesame oil, oliveoil, sunflower oil, palm kernel oil, camellia oil, jojoba oil, macadamianut oil, and tung oil. For example, commercial products available fromIdemitsu Kosan Co., Ltd., Sankyo Yuka Kogyo K. K., Japan EnergyCorporation, Olisoy, H&R, Hokoku Corporation, Showa Shell Sekiyu K. K.,Fuji Kosan Co., Ltd., and The Nisshin OilliO Group, Ltd. may be used.

Examples of the liquid diene polymers include diene polymers which areliquid at 25° C., such as liquid styrene-butadiene copolymers (liquidSBR), liquid polybutadiene polymers (liquid BR), liquid polyisoprenepolymers (liquid IR), liquid styrene-isoprene copolymers (liquid SIR),liquid styrene-butadiene-styrene block copolymers (liquid SBS blockpolymers), liquid styrene-isoprene-styrene block copolymers (liquid SISblock polymers), liquid farnesene polymers, and liquidfarnesene-butadiene copolymers. The chain end or backbone of thesepolymers may be modified with polar groups.

(Other Materials)

The rubber composition may contain a solid resin (resin that is solid atroom temperature (25° C.)) other than the terpene-based resins. Examplesof the solid resin include aromatic vinyl polymers, coumarone-indeneresins, coumarone resins, indene resins, phenolic resins, rosin resins,petroleum resins, and acrylic resins. The combined amount of theterpene-based resin and the solid resin (additional resin) other thanthe terpene-based resins, if present, is preferably 0.1 to 100 parts bymass, more preferably 10 to 70 parts by mass per 100 parts by mass ofthe rubber component.

In view of crack resistance, ozone resistance, and other properties, therubber composition preferably contains an antioxidant.

Non-limiting examples of the antioxidant include: naphthylamineantioxidants such as phenyl-α-naphthylamine; diphenylamine antioxidantssuch as octylated diphenylamine and4,4′-bis(α,α′-dimethylbenzyl)diphenylamine; p-phenylenediamineantioxidants such as N-isopropyl-N′-phenyl-p-phenylenediamine,N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, andN,N′-di-2-naphthyl-p-phenylenediamine; quinoline antioxidants such as2,2,4-trimethyl-1,2-dihydroquinoline polymer; monophenolic antioxidantssuch as 2,6-di-t-butyl-4-methylphenol and styrenated phenol; and bis-,tris-, or polyphenolic antioxidants such astetrakis[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)-propionate]methane.Among these, p-phenylenediamine or quinoline antioxidants are preferred,with N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine or2,2,4-trimethyl-1,2-dihydroquinoline polymer being more preferred. Forexample, commercial products available from Seiko Chemical Co., Ltd.,Sumitomo Chemical Co., Ltd., Ouchi Shinko Chemical Industrial Co., Ltd.,and Flexsys may be used.

The amount of the antioxidant per 100 parts by mass of the rubbercomponent is preferably 0.2 parts by mass or more, more preferably 0.5parts by mass or more. When the amount is not less than the lower limit,sufficient ozone resistance tends to be obtained. The amount is alsopreferably 7.0 parts by mass or less, more preferably 4.0 parts by massor less. When the amount is not more than the upper limit, a good tireappearance tends to be obtained.

The rubber composition preferably contains stearic acid. The amount ofthe stearic acid per 100 parts by mass of the rubber component ispreferably 0.5 to 10 parts by mass, more preferably 0.5 to 5 parts bymass.

The stearic acid may be a conventional one, and examples includeproducts available from NOF Corporation, Kao Corporation, Wako PureChemical Industries, Ltd., and Chiba Fatty Acid Co., Ltd.

The rubber composition preferably contains zinc oxide. The amount of thezinc oxide per 100 parts by mass of the rubber component is preferably0.5 to 10 parts by mass, more preferably 1 to 5 parts by mass.

The zinc oxide may be a conventional one, and examples include productsavailable from Mitsui Mining & Smelting Co., Ltd., Toho Zinc Co., Ltd.,HakusuiTech Co., Ltd., Seido Chemical Industry Co., Ltd., and SakaiChemical Industry Co., Ltd.

The rubber composition may contain a wax. Non-limiting examples of thewax include petroleum waxes and natural waxes, as well as syntheticwaxes produced by purifying or chemically treating a plurality of waxes.These waxes may be used alone or in combinations of two or more.

Examples of the petroleum waxes include paraffin waxes andmicrocrystalline waxes. Any natural wax derived from non-petroleumresources may be used, and examples include plant waxes such ascandelilla wax, carnauba wax, Japan wax, rice bran wax, and jojoba wax;animal waxes such as beeswax, lanolin, and spermaceti; mineral waxessuch as ozokerite, ceresin, and petrolatum; and purified products of theforegoing waxes. For example, commercial products available from OuchiShinko Chemical Industrial Co., Ltd., Nippon Seiro Co., Ltd., and SeikoChemical Co., Ltd. may be used. The amount of the wax may beappropriately selected in view of ozone resistance and cost.

The rubber composition preferably contains sulfur in order to moderatelycrosslink the polymer chains, thereby providing good properties.

The amount of the sulfur per 100 parts by mass of the rubber componentis preferably 0.1 parts by mass or more, more preferably 0.5 parts bymass or more, still more preferably 0.7 parts by mass or more, but ispreferably 6.0 parts by mass or less, more preferably 4.0 parts by massor less, still more preferably 3.0 parts by mass or less. When theamount is within the range indicated above, good properties tend to beobtained.

Examples of the sulfur include those commonly used in the rubberindustry, such as powdered sulfur, precipitated sulfur, colloidalsulfur, insoluble sulfur, highly dispersible sulfur, and soluble sulfur.For example, commercial products available from Tsurumi ChemicalIndustry Co., Ltd., Karuizawa Sulfur Co., Ltd., Shikoku ChemicalsCorporation, Flexsys, Nippon Kanryu Industry Co., Ltd., and HosoiChemical Industry Co., Ltd. may be used. These may be used alone or incombinations of two or more.

The rubber composition preferably contains a vulcanization accelerator.

The amount of the vulcanization accelerator is not particularly criticaland may be freely selected according to the desired cure rate orcrosslink density. The amount is usually 0.3 to 10 parts by mass,preferably 0.5 to 7 parts by mass per 100 parts by mass of the rubbercomponent.

Any type of vulcanization accelerator may be used including thosecommonly used. Examples of such vulcanization accelerators includethiazole vulcanization accelerators such as 2-mercaptobenzothiazole,di-2-benzothiazolyl disulfide, andN-cyclohexyl-2-benzothiazylsulfenamide; thiuram vulcanizationaccelerators such as tetramethylthiuram disulfide (TMTD),tetrabenzylthiuram disulfide.(TBzTD), and tetrakis(2-ethylhexyl) thiuramdisulfide (TOT-N); sulfenamide vulcanization accelerators such asN-cyclohexyl-2-benzothiazole sulfenamide, N-t-butyl-2-benzothiazolylsulfenamide, N-oxyethylene-2-benzothiazole sulfenamide, andN,N′-diisopropyl-2-benzothiazole sulfenamide; and guanidinevulcanization accelerators such as diphenylguanidine,diorthotolylguanidine, and orthotolylbiguanidine. These may be usedalone or in combinations of two or more. Preferred among these aresulfenamide or guanidine vulcanization accelerators.

In addition to the above-mentioned components, the rubber compositionmay appropriately contain other compounding agents commonly used in thetire industry such as release agents.

The rubber composition may be prepared by common methods. For example,the rubber composition may be prepared by kneading the components in arubber kneading machine such as a Banbury mixer, a kneader, or an openroll mill, and vulcanizing the kneaded mixture.

The kneading conditions are as follows. In a base kneading step thatincludes kneading additives other than vulcanizing agents andvulcanization accelerators, the kneading temperature is usually 50 to200° C., preferably 80 to 190° C., and the kneading time is usually 30seconds to 30 minutes, preferably one minute to 30 minutes. In a finalkneading step that includes kneading vulcanizing agents andvulcanization accelerators, the kneading temperature is usually 100° C.or lower, preferably from room temperature to 80° C. The compositionobtained after kneading the vulcanizing agents and vulcanizationaccelerators is usually vulcanized by, for example, press vulcanization.The vulcanization temperature is usually 120 to 200° C., preferably 140to 180° C.

The rubber composition is suitable for treads of tires and particularlyfor treads (monolayer treads, or cap treads of multilayer treads) ofstudless winter tires.

In view of the balance between snow and ice performance and handlingstability, and other properties, the rubber composition (vulcanizedrubber composition) preferably has an E* (complex modulus) at 0° C. of3.0 to 8.0 MPa. The lower limit of the E* is more preferably 3.5 MPa ormore.

In view of the balance between snow and ice performance and abrasionresistance, and other properties, the rubber composition (vulcanizedrubber composition) preferably has a difference between the E* (complexmodulus) at −10° C. and the E* at 10° C. ((E* at −10° C.)−(E* at 10°C.)) of 8.0 MPa or less, more preferably 7.0 MPa or less.

(Pneumatic Tire)

The pneumatic tire of the present invention may be produced using therubber composition by usual methods. Specifically, the rubbercomposition containing the above-mentioned components, beforevulcanization, may be extruded into the shape of a tread (e.g. a captread) and assembled with other tire components on a tire buildingmachine in a usual manner to build an unvulcanized tire, which may thenbe heated and pressurized in a vulcanizer to produce a tire. The tire ofthe present invention is suitable for use as a studless winter tire,especially for passenger vehicles.

The tread of the pneumatic tire thus prepared preferably has a surfacewith pores. Such pores result from dissolution of the water-soluble fineparticle during running under wet road conditions, or are formed by thefoaming agent. The presence of pores on the tread surface makes therubber flexible to improve low-temperature grip performance. Further,the cells or pores in the rubber can drain water to improve snow and iceperformance.

The pores on the surface of the tread preferably have a pore size of 300μm or less, more preferably 200 μm or less, still more preferably 150 μmor less. The lower limit of the pore size is not particularly critical,but is preferably 1 μm or more, more preferably 10 μm or more.

The “pore size” refers to the maximum pore size that may be measured byobserving the tread surface with a transmission electron microscope(TEM). For example, it is the length of the longer diagonal line for arhomboid image or the diameter for a circular (spherical) image.

EXAMPLES

The present invention will be specifically described below withreference to, but not limited to, examples.

[Measurement of Median Particle Size (Median Size) of Water-Soluble FineParticle]

The median particle size of water-soluble fine particles was measured bylaser diffraction using SALD-2000J (Shimadzu Corporation) as describedbelow.

<Measurement Procedure>

The water-soluble fine particles were dispersed in a solution mixture ofa dispersion solvent (toluene) and a dispersant (a 10% by mass solutionof sodium di(2-ethylhexyl) sulfosuccinate in toluene) at roomtemperature. The dispersions were stirred for five minutes underultrasonic irradiation to prepare test solutions. The test solutionswere transferred to a batch cell, and one minute later the measurementwas performed (refractive index: 1.70-0.20 i).

Synthesis Example 1, Synthesis of Modified BR

A 5 L autoclave purged with nitrogen was charged with 2.4 kg ofcyclohexane and 300 g of 1,3-butadiene. Then, a catalyst composition(iodine atom/lanthanoid-containing compound molar ratio=2.0) wasintroduced into the autoclave, and a polymerization reaction was thenperformed for two hours at 30° C. to give a polymer solution. Thecatalyst composition was previously prepared by reacting and aging asolution of 0.18 mmol of neodymium versatate in cyclohexane, a solutionof 3.6 mmol of methylalumoxane in toluene, a solution of 6.7 mmol ofdiisobutylaluminum hydride in toluene, and a solution of 0.36 mmol oftrimethylsilyl iodide (hereinafter, also referred to as “Me₃SiI”) intoluene with 0.90 mmol of 1,3-butadiene for 60 minutes at 30° C. To thepolymer solution kept at a temperature of 30° C. was added a solution of1.71 mmol of 3-glycidoxypropyltrimethoxysilane in toluene, and they werereacted for 30 minutes to give a reaction solution. To the reactionsolution was added a solution of 1.28 mmol of tetraisopropyl titanate(hereinafter, also referred to as “IPOTi”) in toluene, followed bystirring for 30 minutes. Then, a solution of 1.5 g of2,4-di-tert-butyl-p-cresol in methanol was added to terminate thepolymerization reaction. The resultant solution was used as a modifiedpolymer solution (yield: 2.5 kg). To the modified polymer solution wasadded 20 L of an aqueous solution having a pH of 10 adjusted with sodiumhydroxide, followed by solvent removal and a simultaneous condensationreaction at 110° C. for two hours. The resulting product was dried onrolls at 110° C. to obtain a high-cis BR having a polar groupinteractive with silica.

The modified BR thus prepared had a cis content of 96% by mass, a transcontent of 4% by mass, and a Mw of 600,000.

The chemicals used in examples and comparative examples are listedbelow.

NR: RSS#3

Modified BR: the modified BR synthesized in Synthesis Example 1(high-cis modified BR having a polar group interactive with silica)

Carbon black: Seast N220 available from Mitsubishi Chemical Corporation

Silica: Ultrasil VN3 (N₂SA: 172 m²/g) available from Evonik Degussa

Silane coupling agent: Si266 available from Evonik Degussa

Terpene-based resin 1: α-pinene homopolymer (softening point: 130° C.,Mn: 742 g/mol, Mz: 1538 g/mol, Mw: 1055 g/mol, Mw/Mn: 1.42, Tg: 81° C.,limonene unit content: 0% by mass)

Terpene-based resin 2: pinene polymer (α-pinene content: 90% by mass,β-pinene content: 10% by mass, softening point: 130° C., Mn: 657 g/mol,Mz: 1332 g/mol, Mw: 917 g/mol, Mw/Mn: 1.40, Tg: 80° C., limonene unitcontent: 0% by mass)

Terpene-based resin 3: pinene polymer (α-pinene content: 20% by mass,β-pinene content: 80% by mass, softening point: 130° C., Mn: 790 g/mol,Mz: 1891 g/mol, Mw: 1101 g/mol, Mw/Mn: 1.57, Tg: 78° C., limonene unitcontent: 0% by mass)

Resin 4: α-methylstyrene resin (Sylvares SA120 available from KratonPolymers, softening point: 120° C.)

Water-soluble fine particle 1: MN-00 available from Umai Chemical Co.,Ltd. (magnesium sulfate, median particle size (median size): 75 μm)

Water-soluble fine particle 2: USN-00 available from Umai Chemical Co.,Ltd. (extremely fine particle magnesium sulfate, median particle size(median size): 3 μm)

Water-soluble fine particle 3: sodium lignosulfonate available fromTokyo Chemical Industry Co., Ltd. (median particle size (median size):100 μm)

Wax: Ozoace wax available from Nippon Seiro Co., Ltd.

Antioxidant: NOCRAC 6C available from Ouchi Shinko Chemical IndustrialCo., Ltd.

Oil: PS-32 (mineral oil) available from Idemitsu Kosan Co., Ltd.

Stearic acid: “KIRI” available from NOF Corporation

Zinc oxide: zinc oxide #2 available from Mitsui Mining & Smelting Co.,Ltd.

Sulfur: powdered sulfur available from Tsurumi Chemical Industry Co.,Ltd.

Vulcanization accelerator: NOCCELER NS available from Ouchi ShinkoChemical Industrial Co., Ltd.

Examples and Comparative Examples

The natural rubber plus silica, or polybutadiene rubber plus silica wereadded in the amounts listed in Table 1 to a 1.7 L Banbury mixer andkneaded at 150° C. for three minutes to give a kneaded mixture(masterbatch). To the combined masterbatches were added the materialsother than the sulfur and vulcanization accelerator, and they werekneaded at 150° C. for two minutes to give a kneaded mixture. Thekneaded mixture was further kneaded with the sulfur and vulcanizationaccelerator using an open roll mill at 80° C. for five minutes to obtainan unvulcanized rubber composition.

The unvulcanized rubber compositions prepared as above were vulcanizedat 170° C. for 15 minutes to obtain vulcanized rubber compositions(specimens).

Separately, the unvulcanized rubber compositions prepared as above wereeach formed into a cap tread shape and assembled with other tirecomponents, followed by vulcanization at 170° C. for 15 minutes toprepare a test studless winter tire (tire size: 195/65R15).

The vulcanized rubber compositions (specimens) and test studless wintertires prepared as above were stored at room temperature in a dark placefor three months and thereafter evaluated as follows. Table 1 shows theresults.

<Ice Performance>

The performance of the test studless winter tires mounted on a car wasevaluated on ice under the following conditions.

The test site was the Nayoro test track of Sumitomo Rubber Industries,Ltd. in Hokkaido, Japan. The air temperature was −5 to 0° C. The testtires were mounted on a front-engine, rear-wheel-drive car of 2000 ccdisplacement made in Japan. The distance required for the car travelingon ice to stop after the brakes that lock up were applied at 30 km/h wasmeasured.

Ice performance indexes were calculated using the equation below, withComparative Example 1 taken as reference. A higher index indicatesbetter ice performance.(Ice performance index)=(Brake stopping distance of Comparative Example1)/(Stopping distance of each formulation example)×100<Handling Stability>

Each set of test studless winter tires were mounted on the wheels of atest car (a front-engine, front-wheel-drive car of 2000 cc displacementmade in Japan). A test driver drove the car in a zig-zag fashion andthen subjectively evaluated the stability of steering control. Theresults are expressed as an index, with Comparative Example 1 set equalto 100. A higher index indicates better handling stability.

<Rolling Resistance (Fuel Economy)>

The rolling resistance of the test studless winter tires was measuredwith a rolling resistance tester by running each tire mounted on a 15×6JJ rim at an internal pressure of 230 kPa, a load of 3.43 kN, and aspeed of 80 km/h. The reciprocals of rolling resistances are expressedas an index, with Comparative Example 1 set equal to 100. A higher indexindicates a lower rolling resistance and thus better fuel economy.

<Abrasion Resistance>

The abrasion loss of the vulcanized rubber compositions was measuredusing a Lambourn abrasion tester (Iwamoto Seisakusho Co., Ltd.) at asurface rotational speed of 50 m/min, an applied load of 3.0 kg, a sandfall rate of 15 g/min, and a slip ratio of 20%, and a reciprocal of theabrasion loss was calculated. In the table, the reciprocal of theabrasion loss of Comparative Example 1 is set equal to 100, and thereciprocals of the abrasion losses of the other formulation examples areexpressed as an index. A higher index indicates better abrasionresistance.

TABLE 1 Comparative Example Example 1 2 3 4 5 1 2 3 Amount NR 40 40 4040 40 40 40 40 (parts by mass) Modified BR 60 60 60 60 60 60 60 60Carbon black 5 5 5 5 5 5 5 5 Silica 55 55 55 55 55 55 55 55 Silanecoupling agent 4 4 4 4 4 4 4 4 Terpene-based resin 1 25 25 25 25 25 — —25 Terpene-based resin 2 — — — — — — — — Terpene-based resin 3 — — — — —25 — — Resin 4 — — — — — — 25 — Water-soluble fine particle 1 40 30 20 —— 40 40 — Water-soluble fine particle 2 — — — 40 — — — — Water-solublefine particle 3 — — — — 40 — — — Wax 1 1 1 1 1 1 1 1 Antioxidant 2 2 2 22 2 2 2 Oil 25 25 25 25 25 25 25 25 Stearic acid 1 1 1 1 1 1 1 1 Zincoxide 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulfur 1.2 1.2 1.2 1.2 1.2 1.2 1.21.2 Vulcanization accelerator 2 2 2 2 2 2 2 2 Evaluation Ice performance110 107 104 119 108 100 94 84 Handling stability 108 106 106 107 103 10099 95 Rolling resistance 107 110 113 110 104 100 93 95 Abrasionresistance 107 117 125 110 107 100 85 100

As shown in Table 1, the balance of snow and ice performance, wet gripperformance, abrasion resistance, and handling stability wassignificantly improved in the examples in which predetermined amounts ofa terpene-based resin having predetermined softening point and α- andβ-pinene unit contents, and a water-soluble fine particle and/or afoaming agent were added to a composition containing an isoprene-basedrubber, polybutadiene rubber, silica, and carbon black.

The invention claimed is:
 1. A pneumatic tire, comprising a tread, thetread comprising a rubber composition for treads comprising: a rubbercomponent comprising an isoprene-based rubber and polybutadiene rubber;silica; carbon black; a terpene-based resin; and a water-soluble fineparticle, the terpene-based resin having a softening point of 70 to 150°C., an α-pinene unit content of 65 to 100% by mass, and a β-pinene unitcontent of 0 to 35% by mass, the rubber component comprising, based on100% by mass thereof, 20 to 80% by mass of the isoprene-based rubber and20 to 80% by mass of the polybutadiene rubber, with a combined amount ofthe isoprene-based rubber and the polybutadiene rubber of 90% by mass ormore, the rubber composition comprising, per 100 parts by mass of therubber component, 20 to 200 parts by mass of the silica, 0.1 to 50 partsby mass of the carbon black, and 0.1 to 50 parts by mass of theterpene-based resin.
 2. The pneumatic tire according to claim 1, whereinthe terpene-based resin is at least one selected from the groupconsisting of α-pinene homopolymers, terpene-based resins having anα-pinene unit content of 99% by mass or higher, and copolymerscontaining an α-pinene unit and a β-pinene unit.
 3. The pneumatic tireaccording to claim 1, wherein the water-soluble fine particle has amedian particle size of 0.1 to 500 μm.
 4. The pneumatic tire accordingto claim 1, wherein the water-soluble fine particle is at least oneselected from the group consisting of magnesium sulfate, potassiumsulfate, sodium chloride, calcium chloride, magnesium chloride,potassium carbonate, sodium carbonate, lignin derivatives, andsaccharides.
 5. The pneumatic tire according to claim 1, wherein thepolybutadiene rubber has a cis content of 90% by mass or higher.
 6. Thepneumatic tire according to claim 1, wherein the polybutadiene rubber isa modified polybutadiene rubber that has been modified with a polargroup interactive with silica.
 7. The pneumatic tire according to claim1, wherein the silica has a nitrogen adsorption specific surface area of160 to 300 m²/g.
 8. The pneumatic tire according to claim 1, wherein therubber composition has an E* at 0° C. of 3.0 to 8.0 and a differencebetween E* at −10° C. and E* at 10° C. of 7.0 or less.
 9. The pneumatictire according to claim 1, wherein the tread has a surface with poreshaving a pore size of 300 μm or less.
 10. The pneumatic tire accordingto claim 1, wherein the pneumatic tire is a studless winter tire. 11.The pneumatic tire according to claim 1, wherein the amount of a liquidplasticizer per 100 parts by mass of the rubber component is 25 parts bymass or less.
 12. The pneumatic tire according to claim 1, wherein thewater-soluble fine particle is magnesium sulfate.